1. Field of the Invention
The present invention is directed to a hydraulic control device for controlling composite operations of an excavator and, more specifically, to a hydraulic control device for controlling a boom-swing frame combined motion that increases a swing torque to facilitate the swing motion of a swing frame when an excavator performs composite operations such as a loading-on-truck operation, a ground leveling operation and the like through the combined motion of a boom and a swing frame.
2. Description of the Related Art
Hydraulic excavators are provided with front operation components including a boom, an arm and a bucket, which components can be actuated independently or in combination by a hydraulic flow discharged from one or more fluid pump to conduct a variety of composite operations such as a digging operation, a ground leveling operation, a loading-on-truck operation and the like. As used herein, the term “composite operations” refers to excavator operations that are performed by simultaneous actuation of two or more of a boom cylinder, an arm cylinder and a swing motor.
As is known in the art, the conventional hydraulic excavators include a hydraulic control device. The hydraulic control device is provided with a control valve having boom first-speed and second-speed control spools and arm first-speed and second-speed control spools that can be shifted to cause a boom and an arm to move at a relatively low speed (“first speed”) or a relatively high speed (“second speed”) depending on the kind of operations to be carried out.
The boom second-speed control spool and the arm first-speed control spool, which belong to a first spool group, are in fluid communication with a primary fluid pump, while the boom first-speed control spool and the arm second-speed control spool, which belong to a second spool group, are in fluid communication with a secondary fluid pump. The hydraulic flow generated by only one of the fluid pumps is used in actuating a boom cylinder or an arm cylinder at the first speed. On the contrary, the hydraulic flows generated by the primary pump and the secondary pump are combined together in order to actuate the boom cylinder or the arm cylinder at the second speed.
More specifically, referring to FIG. 1, the prior art hydraulic control device is provided with a first fluid pump P1 and a second fluid pump P2 each for generating a hydraulic flow to be used in the hydraulic control device. The hydraulic control device is further provided with a control valve 100 for controlling the hydraulic flows of the first and second fluid pumps P1, P2 supplied to the boom cylinder 105 and the arm cylinder 107. The control valve 100 includes a first control spool group 100A consisting of a boom second-speed control spool 101B and an arm first-speed control spool 103A, and a second control spool group 100B consisting of a boom first-speed control spool 101A and an arm second-speed control spool 103B. Also included in the first spool group 100A is a swing control spool 109 for controlling actuation of a swing motor 120.
In other words, the swing control spool 109, the boom second-speed control spool 101B and the arm first-speed control spool 103A are respectively provided on a first bypass line 110 from upstream to downstream in the named sequence for receiving the hydraulic flow from the first pump P1 via parallel lines 111, 112, 113. The boom first-speed control spool 101A and the arm second-speed control spool 103B are respectively provided on a second bypass line 115 from upstream to downstream in the named sequence for receiving the hydraulic flow from the second pump P2 via the parallel lines 116, 117.
Referring again to FIG. 1, the hydraulic flow of the first fluid pump P1 is distributed to the swing control spool 109 and the boom second-speed control spool 101B to thereby actuate the swing motor 120 and the boom cylinder 105 at the second speed. and The boom second-speed control spool 101B and the boom first-speed control spool 101A are connected to the boom cylinder 105 via a confluence line 112 so that the hydraulic flow of the first fluid pump P1 and the hydraulic flow of the second fluid pump P2 can be merged together in the confluence line 112 after passing through the boom second-speed control spool 101B and the boom first-speed control spool 101A, respectively. A check valve 123 is disposed on the confluence line 112 in such a manner that the check valve 123 allows the hydraulic flows passed the boom second-speed control spool 101B to be supplied to the boom cylinder 105 through the confluence line 112 but prevents the hydraulic flow passed the boom first-speed control spool 101A from flowing toward the boom second-speed control spool 101B.
In this type of hydraulic control device, the swing actuation pressure is greater than the boom actuation pressure at an initial stage of the boom-swing frame combined motion during which the boom cylinder 105 and the swing motor 120 are actuated simultaneously. This causes the hydraulic flow of the first fluid pump P1 to be first supplied to the boom cylinder 105, thus balancing the actuation pressure of the boom cylinder 105 with that of the swing motor 120.
The swing motor 120 is associated with a relief valve 125 that serves to limit the pressure of the hydraulic flow supplied to the swing motor to below a predetermined value. In case that a swing frame alone is caused to move by the swing motor 120 with other operating components left inoperative, the swing actuation pressure is increased up to a relief pressure of the relief valve 125 as illustrated by a line A1 in FIG. 2a and therefore the swing torque is also maximized as illustrated by a line A2 in FIG. 2b, thus enabling the swing operation to take place in a smooth manner.
In case of a boom-swing frame combined motion, however, the swing actuation pressure does not soar up to the maximum value but remains equal to the boom actuation pressure as indicated by a line B1 in FIG. 2a, which means that the swing torque fails to reach the maximum value as indicated by a line B2 in FIG. 2b. This makes it difficult to move the swing frame which requires greater torque than the boom during the course of boom-swing frame composite operations.